The present invention relates to improvements in automatic telephone answering systems; and more particularly, the present invention relates to apparatus for detecting dial tone on the telephone line after the calling party hangs up, and in response thereto disconnecting from the telephone line.
At the present time there are various types of systems which automatically answer an incoming call and hold a telephone line connection which are broadly defined as "automatic telephone answering systems". Included in this category of apparatus are automatic telephone message recording equipment, automatic telephone call forwarding equipment, automatic dial-up alarm receiving equipment, and automatic answering data entry systems. In each of these systems, some means is included for establishing a connection to the telephone line. Typically, closure of a line seizure relay is utilized to make the connection upon the occurrence of an incoming call. Disconnection from the telephone line is accomplished by opening the line seizure relay in response to a condition interpreted as calling party hanging up.
It has been common practice in prior art automatic telephone answering systems, particularly automatic telephone message recording equipment, to control the opening of the line seizure relay with a "calling party control" (CPC) relay. The CPC relay is so interconnected with the line seizure relay that while a calling party maintains his telephone set in an "off-hook" condition, electrical current exists in the CPC relay coil and the line seizure relay maintains a connection to the telephone line. When the calling party hangs up, the CPC relay coil becomes deenergized, with resulting disconnection from the telephone line. This manner of disconnecting from the telephone line upon hang up of a calling party relies upon the detection of breaks in the subscriber loop-current.
Difficulty arises, however, with loop-current break detection of caller hang up in that apparatus is required which is capable of sensing breaks as short as 8 milliseconds. Despite the utilization of elaborate loop-current break sensing apparatus, detection of loop-current breaks and disconnection from the telephone line is frequently much later than the actual point of caller hangup at the point of switching to the central-offices "permanent signal" buss.
An alternative is to detect dial-tone and disconnect in response thereto. In the prior art dial-tone detection has been by a tunable tone receiver added to the system. Effective dial tone detection requires a tone receiver which does not falsely indicate a dial tone detection from other line signals, such as voice signals. Thus, dial tone receivers by necessity comprise complex filter circuitry and, as a result, add measurably to the necessary system hardware.
Also, disconnection from the telephone line in response to the detection of dial tone with a tone receiver, regardless of the sophistication of the circuit design, does not provide a safeguard against inadvertent disconnection in response to transient signals appearing on the line. Furthermore, in a message recording system having remote access message playback, dial tone detection by a tone receiver is susceptible to causing an inadvertent disconnect in response to simulated dial tone (i.e., dial tone previously recorded on the message tape and output during playback).